1. Field of the Invention
This invention relates to an oxide glass capable of exhibiting a long lasting afterglow and photostimulated luminescence, whereby energy can be accumulated by radiation excitation, for example, by .gamma.-rays, X-rays, UV-rays, etc., light emission can be continued for a long time even after stopping the excitation, so that the oxide glass can be used as a phosphorescent material for night illumination or night signal, and furthermore, photostimulated luminescence can be exhibited by irradiation of visible rays or infrared rays, which is useful for confirming an infrared laser or controlling an optical axis of a laser beam, so that recording or reproducing of .gamma.-rays, X-rays or UV-rays images can be effected, and which can be used as an optical recording material capable of reading and writing.
2. Description of the Prior Art
Phosphorescent materials or photostimulated luminescence phosphor using rare earth elements have widely been used up to the present time. As a phosphorescent material, there can practically be used SrAl.sub.2 O.sub.4 :Eu.sup.2+, Dy.sup.3+, etc. and as a photostimulated luminescence phosphor, there has already been developed BaFBr:Eu.sup.2+, etc. These are opaque materials only exhibiting superficial phosphorescence, generally obtained by coating a suitable carrier with a powdered phosphorescent material or photostimulated luminescence phosphor.
On the other hand, a glass capable of exhibiting a photostimulated luminescence without coating a photostimulated luminescence phosphor has been disclosed in Japanese Patent Laid-Open Publication No. 221336/1997. In addition, Japanese Patent Application Nos. 346362/1997 and 88674/1998 respectively disclose obtaining phosphorescent glasses of SiO.sub.2 --B.sub.2 O.sub.3 --ZnO--Tb.sub.2 O.sub.3 system and SiO.sub.2 --Ga.sub.2 O.sub.3 --Na.sub.2 O--Tb.sub.2 O.sub.3 system.
In the case of coating a powdered phosphorescent material or photostimulated luminescent phosphor, dispersion of the coating thickness or peeling-off of the coated surface occurs whereby to cause light and shade of the phosphorescence. In order to increase the emission intensity, it is required to coat a phosphorescent material or photostimulated luminescence phosphor with a larger thickness, but the coating is limited by increase of opaqueness thereof. Since a phosphorescence emitted from a grain boundary in the phosphorescent material or photostimulated luminescence phosphor is scattered, furthermore, no clear image can be obtained.
A photostimulated luminescent glass described in Japanese Patent Laid-Open Publication No. 221336/1997 utilizes Ce.sup.3+ or Eu.sup.2+ as an active ion for emission. In order to obtain these active ions, it is necessary to add a reducing agent or to melt in a reducing atmosphere. Furthermore, observation of the emission is difficult because of being UV light or blue light.